Power generation systems often include a power converter that is configured to convert an input power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system, such as a wind turbine system, may include a power converter for converting variable frequency alternating current power generated by the wind turbine system into alternating current power at a grid frequency (e.g. 50 Hz or 60 Hz) for application to a utility grid. An exemplary power generation system may generate AC power using a wind-driven doubly fed induction generator (DFIG), as known in the art. A power converter associated with the DFIG can be used to regulate the flow of electrical power between the DFIG and the grid or other load.
In other power generating wind turbine systems, for example, a so-called full power conversion system, conversion systems are also employed including a power converter for converting variable frequency alternating current power generated by the wind turbine system into alternating current power at a grid frequency. Both the DFIG and full power conversion systems employ an AC-DC-AC conversion topology.
In still other power generating systems, for example, solar power generating systems, converters are also employed, for example, two-stage solar inverters, which have a DC-DC-AC inverter topology and are referred to herein as solar inverters.
DFIG Wind Turbine Systems are well known with converters that do not include an energy storage system. New systems, however, are now being developed that utilize a battery for energy storage. Examples of such systems are shown in FIGS. 3 and 4. These systems generally include a battery and a bi-directional DC-DC switching power supply to transfer energy back and forth between the DC bus of the AC-DC-AC Converter and a large battery bank, as illustrated in FIG. 5. In addition to functioning as a battery charge controller, controlling the power flow to and from the battery, the bi-directional DC-DC switching power supply is also used to regulate the voltage of the battery at a level appropriate for the battery, which voltage level may not correspond to the same voltage as that of the DC Bus of the AC-DC-AC Converter or, in the case of a solar inverter, the DC-DC-AC converter. All of these systems have a large power converter with a regulated DC Bus.
Accordingly, a system and method for operating a power generation system that allows both storage and use of battery energy as a part of the alternating energy system is desirable. In such a system, it would be particularly desirable to avoid, or at least limit, common mode and normal mode voltages being applied to the energy storage battery incorporated into such alternating energy system.